Glass sheets are manufactured by vertically drawing a sheet from a molten glass mass, by processes known variously as Fourcault, Asahi, Colburn, P.P.G., etc.
In said processes, the molten glass mass passes from the furnace to a refining channel which ends at a vertical drawing chamber. The glass flows vertically through a draw bar or rolls (Asahi process) or is drawn from the free surface of the glass mass (P.P.G. process) wherein the temperature is of about 1000.degree. C.
As the glass sheet is drawn by a series of pairs of rolls from above the glass mass, it is rapidly cooled by means of heat exchangers in order to prevent the rolls from marking the surface of the glass sheet and to produce a finish called fire finishing on the surface of the glass sheet.
The average thickness of the glass sheet at the drawing line depends on the drawing speed and temperature of the glass in the meniscus. Control of the thickness variation throughout the glass sheet, depends mainly on the control of the temperature along the meniscus and this is achieved in the previous art by means of heat exchangers and adjacent refractories.
At present, all vertical drawing machines can not produce glass sheets of a quality comparable to that of the float glass.
The glass quality is related with the degree of distortion present in the glass sheets (the greater the distortion the lesser the quality).
U.S. Pat. No. 3,248,193 of F. R. Hohmann, describes a method for drawing glass, comprising floating an essentially immiscible layer of liquid on the surface of the bath, pool or drawing chamber of molten glass mass, in an inert atmosphere to avoid oxidation of the floating liquid layer, and drawing a sheet of glass through said layer.
The sole objective of including the floating liquid layer, in accordance with said method, is to protect the drawn glass during its formative stage from extraneous forces such as air currents.
However, the real problem for the distortion failures in the glass sheet forming process was never envisioned in the process disclosed in said patent.
Applicants found that the method disclosed in said patent was not commercially implemented because of a limiting factor: the condition under which laboratory experiment was carried out to show the feasibility of said method, was that the whole system had to be at the same temperature. This involved that a total absence of convective currents both in the floating layer and in the glass mass, could be obtained only when the floating liquid layer and the glass mass in the drawing chamber were at the same temperature. In the practice it is hardly possible to operate the method under such conditions, because when one has the glass mass at a temperature different than that of the layer, convective currents hard to control will be generated both in said layer and in the glass mass.
It is well known that one of the causes of distortion in the glass sheets is due to the differences in the thickness thereof.
Applicants have also found that said difference in the thickness of the glass sheet is due that, during the drawing process, convective currents are generated in the molten glass mass because of the differences in the glass homogeneity and in the temperature thereinto, which affect the viscosity thereof and consequently the final thickness of the glass sheet.
Furthermore, applicants found that the viscous forces and the thermal diffusion could oppose said convective currents and therefore, by homogenizing the temperature and viscosity of the molten glass mass, said convective currents could be eliminated or damped.
And last but not least, applicants found that control of the temperature and viscosity homogeneity in the molten glass mass could be obtained by placing a stratified liquid layer having a smaller density than that of the glass, on the bath of molten glass in the vertical drawing chamber, so as to eliminate or dampen the convective currents in the glass mass, providing an homogeneous glass mass which, on applying hydrostatic pressure through the same stratified liquid layer, helps in producing a uniform distribution of the thickness of the glass throughout the glass sheet, thus diminishing the causes of distortion.
Said stratified liquid layer may comprise a single layer having variable density or several layers having different densities appearing as a multiple phase layer.